1. Field:
The present invention relates to methods and apparatus for testing electromagnetic apparatus and, in particular, for testing D.C. motors, D.C. generators and other apparatus utilizing electromagnetic windings.
2. State of the Art:
In the manufacture of as well as the repair of D.C. motors and generators it is necessary to determine the neutral point or neutral plane of the motor or generator. The neutral point or neutral plane of a D.C. motor or generator can be defined as a particular position in which the brushes are positioned relative to the field poles such that a minimum of sparking or arcing is produced at the commutator bars. At the neutral point or neutral plane, the armature coils, which are shorted by the brushes through the commutator, are positioned at or almost exactly between stator poles of opposite magnetic polarity. These armature coils cut relatively few lines of magnetic flux, and there is minimal current induced in the armature coils. Therefore, only minimal if any sparking occurs at the commutator when the armature coils are shorted together by the brushes.
Proper setting of the neutral plane is critical when manufacturing or repairing D.C. motors and generators inasmuch as abnormal sparking or arcing at the commutator causes undue, rapid deterioration of the commutator and brushes. Further, accurate setting of the neutral plane is vital to achieving optimum performance and efficiency from the motor or generator.
Prior art methods of setting the neutral plane in D.C. motors and generators are time consuming, and it is difficult to achieve any reasonable degree of accuracy. In one prior art method, the motor or generator is at least partially disassembled. One of the armature coils slots is marked, and the leads from this coil slot is traced to the commutator. This procedure is often complicated inasmuch as the coil slots are often rather inaccessible. After having marked one of the armature coil slots and tracing the leads therefrom to the commutator, the armature is turned so that the marked slot is positioned directly beneath the interpole. The brush holder is then moved to a position in which one brush is over the commutator bars connected to the marked coil. This procedure is time consuming and thus economically inefficient.
A second prior art method that has been utilized for setting the neutral plane of a D.C. motor involves repetitive operation of the motor in both directions under load. The brush holder is adjusted between each set of runs, with the object being to achieve a proper neutral plane setting in which the motor operates at the same speed in both directions. Although this procedure does not require extensive disassembly of the motor, the procedure is excessively time consuming and again economically inefficient.
A third prior art method of setting the neutral plane involves applying a normal current in the armature-interpole circuit without any field current. The brush holder is adjusted in successive tests to achieve a neutral plane setting in which the armature will not turn when the armature-interpole current is applied. This procedure is time consuming and as well involves a rather high degree of danger even when carefully performed by a skilled operator.
Another problem inherent in testing and checking D.C. motors and generators is encountered in determining and locating common armature faults such as grounded, shorted, faulty or incorrectly connected coils or bars in the armature assembly. The faulty coils or bars are difficult to isolate because any of the numerous bars or coils have electrical continuity with the other bars or coils in the assembled armature. The armature faults, unless isolated and corrected, will cause commutator arcing, poor equipment performance, or complete failure of the equipment. Prior art methods of locating such armature faults are described in "Electric Motor Repairs" by Robert Rosenberg which was published by Holt, Pinehart, and Winston. These methods are very time consuming and the results are often ambiguous.
In manufacturing or repairing D.C. motors and generators, the relative magnetic polarity of the poles in the apparatus must be determined in order to establish the correct rotational direction as well as to facilitate connection to the armature so that when the direction of rotation is selected, the interpoles augment (boost) the action of the main field rather than impede (buck) the main fields when the apparatus is operating. Failure to correctly connect the armature circuit at the brush holder or rack will result in extremely inefficient operation or total failure of the apparatus. Prior art methods for determining relative magnetic polarity involve the use of a magnetic compass. Such methods often produce ambiguous results because the compass needle can be deflected under the influence of an adjacent pole when all fields are energized.
Another problem encountered in checking or repairing D.C. motors and generators is to detect shorted or grounded field windings. The detection of shorted or grounded field windings is difficult, especially with series windings, because of the extremely low resistances of the windings. Prior art methods for detecting shorted or grounded field windings involve the use of a low resistance measuring device such as a wheatstone bridge or the use of a voltmeter to detect relative voltage drops across the windings. These methods are inconvenient because stripping or puncturing the insulation around the winding connections is required to perform the test procedures.
3. Objectives:
A principal objective of the present invention is to provide improved apparatus and methods for analyzing various parameters of D.C. motors and generators. A particular objective of the present invention is to provide improved apparatus and method for rapidly, accurately and safely determining the position of the neutral plane in D.C. motors and commutator type generators. An additional objective is to provide improved apparatus and method for locating and isolating grounded, shorted, faulty, open or crossed coils or bars in a D.C. armature. A further objective is to provide improved apparatus and method for determining the relative magnetic polarity of the windings of various electromagnetic devices, including the field windings in D.C. motors, and generators. A still further objective is to provide improved apparatus and method for detecting shorted, grounded or defective windings of various electromagnetic devices, including field coil windings in D.C. motors and generators.